The couple, who run a guesthouse in Marazion, Cornwall, have already lost fights in a county court and the Court of Appeal.

In 2011 a judge concluded that the Bulls acted unlawfully and ordered them to pay a total of £3,600 damages after a hearing at Bristol County Court. In 2012 the Court of Appeal dismissed an appeal by the Bulls following a hearing in London.

The Bulls say they think that any sex outside marriage is ''a sin'' and deny discriminating against Mr Hall and Mr Preddy.

Five Supreme Court justices are scheduled to hear legal argument over two days in London and are expected to reserve a decision until later.

And he said in written submissions given to judges that the case was about the interaction between the "social demands of equality law" and the protection given to individuals under human rights law.

Mr O'Neill said the Bulls' decision had been founded on their "religiously informed judgment of conscience".

"The Bulls are Christian believers. They believe the Bible to be God's word, which reveals God's perfect standards," he said. "They take this responsibility very seriously and always strive to keep their consciences clear before God."

He added: "If the Bulls are required by law to provide double-bedded rooms in their hotel to persons who are not in a monogamous opposite sex marriage with one another, then, in order to remain faithful witnesses to their religious beliefs and true to their religiously informed conscience, they will need to cease operating their hotel and, indeed, to withdraw from providing overnight hospitality to any section of the public."

Robin Allen QC, for Mr Hall and Mr Preddy, disagreed.

"They have lived a life for many years in which as two gay men in an established, long term and committed relationship," he said, in written submissions given to judges.

"They have received less favourable treatment compared to an opposite-sex couple who are married."

The Bulls were at the hearing to see Mr O'Neill outline arguments to judges.